Alisson Discacciati Neves

In situ evaluation of the polymerization kinetics and corresponding evolution of the mechanical properties of dental composites

Polymer composites have been used in dental applications for more than 25 years. Although their properties and behavior have been systematically improved, they still are not able to produce dental restorations chemically, dimensionally and mechanically stable for long periods of time. Low degrees of monomer conversion and poor processing control are some of the main features responsible for the materials instability. In this work, the monomer conversion of dental composites during visible light irradiation was in situ monitored by infrared spectroscopy. The relationship between degree of conversion and mechanical properties was obtained by evaluating the microhardness of composites with different degrees of conversion. The relationship obtained was then used to identify the evolution of the mechanical properties during photopolymerization.

Effect of light intensity and irradiation time on the polymerization process of a dental composite resin

Polymerization shrinkage is a critical factor affecting the longevity and acceptability of dental composite resins. The aim of this work was to evaluate the effect of light intensity and irradiation time on the polymerization process of a photo cured dental composite resin by measuring the Vickers hardness number (VHN) and the volumetric polymerization shrinkage. Samples were prepared using a dental manual light-curing unit. The samples were submitted to irradiation times of 5, 10, 20 and 40 s, using 200 and 400 mW.cm-2 light intensities. Vickers hardness number was obtained at four different moments after photoactivation (immediate, 1 h, 24 h and 168 h). After this, volumetric polymerization shrinkage values were obtained through a specific density method. The values were analyzed by ANOVA and Duncans (p = 0.05). Results showed increase in hardness values from the immediate reading to 1 h and 24 h readings. After 24 h no changes were observed regardless the light intensities or activation times. The hardness values were always smaller for the 200 mW.cm-2 light intensity, except for the 40 s irradiation time. No significant differences were detected in volumetric polymerization shrinkage considering the light intensity (p = 0.539) and the activation time (p = 0.637) factors. In conclusion the polymerization of the material does not terminate immediately after photoactivation and the increase of irradiation time can compensate a lower light intensity. Different combinations between light intensity and irradiation time, i.e., different amounts of energy given to the system, have not affected the polymerization shrinkage.